In recent years, various attempts have been made to know mental activities of human being by analyzing the brain wave of the human brain.
The past analyses and studies revealed that there are sinewave signals after or portions of the brain wave called .alpha., .beta., .delta. and .THETA. rhythms, any one of which is generated or amplified in accordance with the state of mind of an individual person at the time of measurement.
A frequency component of a brain wave ranging from 0.5 Hz to 3.5 Hz is called the .delta. rhythm, that ranging from 3.5 Hz to 7.5 Hz the .THETA. rhythm, that ranging from 7.5 Hz to 13.5 Hz the .alpha. rhythm, and that ranging from 13.5 Hz to 30.5 Hz the .beta. rhythm, respectively.
As means for studying the mental activities of human being, attempts have also been made to analyze the brain wave and measure the degree of mental concentration based on the .alpha. rhythm in the brain wave.
A means for detecting the aforementioned .alpha., .beta., .delta. and .THETA. rhythms has already been proposed by some of the present Applicants (U.S. Patent application Ser. No. 07/794,526). As a means for extracting the .alpha. rhythm of this invention, which will be described later, the means used in the invention for which the aforementioned application was filed may be used as it is.
Conventional mental-concentration measuring methods usually rely upon electrodes pierced into the head of a subject for extracting a signal component representing the .alpha. rhythm alone from among small biological signals generated on the electrodes, and measure the mental concentration of the subject based on an absolute amount obtained by integrating the potentials thereof.
With these conventional methods, however, contact resistance between the electrode and the skin may greatly vary depending on the manner in which the electrodes are pierced into the head, or the state of the skin. As a result, the measured potentials may also vary even when measured simultaneously, as shown in curves A and B in FIG. 1, which are shown in a somewhat exaggerated manner. This could result in failure to precisely reflect the mental activities of human being.